**Ohm:**It is the S.I. unit of resistance. A conductor has a resistance of one ohm if a current of one ampere flows through it on applying a potential difference of one volt across its ends. 1 ohm = 1 volt/1 ampere or 1Ω = 1V/1A**Factors on which resistance of a conductor depends:**The resistance R of a conductor depends

i) Directly on its length L i.e. R α L. ii) inversely on its area of cross-section A i.e. R α 1/A iii) on the nature of material of the conductor on. On combining the above factors, we get R α L/A R = ρ * L/A The proportionality constant ρ is called

resistivity of conductor.

**Resistivity:**It is defined as the resistance offered by a cube of a material of side 1 m when current flows perpendicular to its opposite faces. Its S.I. unit is ohm-meter (Ωm). Resistivity, ρ = RA/L**Equivalent resistance:**If a single resistance can replace the combination of resistances in such a manner that the current in the circuit remains unchanged, then that single resistance is called the equivalent resistance.**Laws of resistances in series:**i) Current through each resistance is same. ii) Total voltage across the combination = Sum of the voltage drops. V= V_{1}+ V_{2}+ V_{3 }iii) Voltage drops across any resistor is proportional to its resistance. V_{1}= IR_{1}, V_{2}= IR_{2}, V_{3}= IR_{3}iv) Equivalent resistance = Sum of the individual resistances. R_{s}= R_{1}+ R_{2}+ R_{3}v) Equivalent resistance is larger than the largest individual resistance.**Laws of resistances in parallel:**i) Voltage across each resistance is same and is equal to the applied voltage. ii) Total current = Sum of the currents through the individual resistances. I = I_{1 }+ I_{2}+ I_{3}iii) Currents through various resistances are inversely proportional to the individual resistances. I_{1}= V/R_{1}, I_{2}= V/R_{2}, I_{3}= V/R_{3}iv) Reciprocal of equivalent resistance = Sum of reciprocals of individual resistances. 1/R_{p }= 1/R_{1}+ 1/R_{2}+ 1/R_{3}v) Equivalent resistance is less than the smallest individual resistance.**Joule’s law of heating:**It states that the heat produced in a conductor is directly proportional to (i) the square of the current I through it (ii) proportional to its resistances R and (iii) the time t for which current is passed. Mathematically, it can be expressed as H = I^{2}Rt joule = I^{2}Rt/4.18 cal or H = VIt joule = VIt/4.18cal**Electric energy:**It is the total work done in maintaining an electric current in an electric circuit for given time.

Electric energy, W = VIt = I^{2}Rt joule

**Electrical power:**Electrical power is the rate at which electric energy is consumed by an appliance.

P = W/t = VI = I^{2}R = V^{2}/R

**Watt:**It is the S.I. unit of power. The power of an appliance is 1 watt if one ampere of current flows through it on applying a potential differences of 1 volt across its ends. 1 watt = 1 joule/1 second =1 volt x 1 ampere or 1 W = 1 Js^{-1}= 1 VA 1 kilowatt = 1000 W 1Mega watt= 10^{6}watt 1Gigawatt=10^{9}watt

**Kilowatt hour:**It is the commercial unit of electrical energy. One kilowatt hour is the electric energy consumed by an appliance of 1000 watts when used for one hour. 1 kilowatt hour (kWh) = 3.6 x 10^{6}J

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